A cum rag is a device used to catch ejaculation fluid after one. A cum rag is usually an old t-shirt, towel, sock, pretty much anything handy of the sorts. In a typical masturbathon, people will use a cum rug. A cum rug is basically an enlarged cum rag.

Guy: Mom! Did you take my cum rag?
Mother: Yeah. It needed washed. It was all crusty.